Why does the cathode bypass cap always use high-voltage in tube amps?

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psychepool
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Why does the cathode bypass cap always use high-voltage in tube amps?

Post by psychepool »

Most tube amplifiers use bulky high-voltage capacitors whenever they are not electrolytic capacitors when applying a cathode bypass cap.
Actually, the voltages applied to the cathode are less than 50v, so why use such bigger ones?
Is there a problem with using a small film capacitor that just gets into the Stompbox effector?
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by frankdrebin »

Because it's MOJO

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Littlewyan
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by Littlewyan »

No real reason other than what they've got to hand. I use 25V capacitors generally.
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by Stevem »

The fact is that when you use a cap of over rated voltage they last longer and higher voltage levels and physical size go hand in hand!

Caps also have a max AC voltage rating ( as in signal level ) that is more times then not equal to 1/2 of its DC voltage rating.
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by chief mushroom cloud »

The smallest 25uF film capacitor I could find (in lowest DC voltage)...has a 37.5mm lead spacing. A .68uF 100V film cap is ~20mm L. 25V and 50V electrolytics are MUCH smaller....
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by pdf64 »

psychepool wrote: Thu Nov 16, 2017 7:26 amMost tube amplifiers use bulky high-voltage capacitors whenever they are not electrolytic capacitors when applying a cathode bypass cap.
I don't understand what you are referring to?
Please can you provide some examples of this?
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by pompeiisneaks »

pdf64 wrote: Thu Nov 16, 2017 6:45 pm
psychepool wrote: Thu Nov 16, 2017 7:26 amMost tube amplifiers use bulky high-voltage capacitors whenever they are not electrolytic capacitors when applying a cathode bypass cap.
I don't understand what you are referring to?
Please can you provide some examples of this?
Yes, I too was confused, most tube amps I've seen either use 25V or 50V electrolytic bypass caps... maybe I'm misunderstanding something?

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xtian
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by xtian »

My guess is Marshall cathode bypass is often 0.68uF, and I've seen 600v caps there, probably because that's what's handy.
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by sluckey »

He is specifically not interested in electrolytic caps. He's talking about smaller caps that you will see in a lot of popular Marshall amps, for example. Like the 0.68µF cathode bypass cap. Those would be huge if using 500 volt caps but they are a very manageable size if only 50v or 100v. I'd like to see some of those amps that are using high voltage caps for that purpose. Low voltage caps are available.
psychepool
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by psychepool »

I'm sorry for the confusion. It's a question that I thought about converting the Marshall diy amplifier.
I was always wondering what the reason for using the high-voltage was like the .680uF cathode bypass cap on the first stage of Marshall.
When the electrolytic capacitor is used as the cathode bypass cap, 25 ~ 50V is usually used, but when it is not electrolytic capacitor, the high voltage is always used. I wondered about this.
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by M Fowler »

Unless one searches for a good source for thos .68, .47 etc. caps at 100v you usually find larger voltage caps instead.

If I order some part from surplus electronic's company I look around to see if they have lower voltage caps not only for Marshall amps but to use in guitars as well.

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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by pompeiisneaks »

psychepool wrote: Fri Nov 17, 2017 6:04 am I'm sorry for the confusion. It's a question that I thought about converting the Marshall diy amplifier.
I was always wondering what the reason for using the high-voltage was like the .680uF cathode bypass cap on the first stage of Marshall.
When the electrolytic capacitor is used as the cathode bypass cap, 25 ~ 50V is usually used, but when it is not electrolytic capacitor, the high voltage is always used. I wondered about this.
Oh gotcha, sorry that's why I was confused, I haven't ever done Marshalls, and didn't realize that. Yeah I think as others have said, they're mostly available in those sizes. I did a search and found some CDE ones that look 'okay' in the 200V range, as an example:

https://www.mouser.com/ProductDetail/Co ... QZsctgo%3d

But most of the caps that are used for coupling need the higher voltage rating, and I think since many people buy them in bulk if they're doing mass installs, it's just cheaper to use the ones they already have in that same rating/group.

Those kinds exist, so go ahead and use them in those builds for sure. For a one off builder it's likely cheaper.

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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by sluckey »

Hoffman sells two brands of low voltage 0.68µF.
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by JMFahey »

Define "low voltage" :wink:
Lowest I found were 160V and 250V; I guess OP was expecting 25V and 50V film capacitors.
It won´t happen.

Main problem is that for lower voltage rating you need atoms thick film ... potential for pores or even holes, also tearing (or at least stretching) is very high , not practical below a certain thickness.

While on electrolytics nobody physically makes, even less winds, any kind of film, dielectric is chemically created and can be real thin.
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Re: Why does the cathode bypass cap always use high-voltage in tube amps?

Post by sluckey »

JMFahey wrote: Fri Nov 17, 2017 5:57 pm Define "low voltage" :wink:
In this specific case I would define low voltage as 160v or 250v. :wink:

Guess we also need to define high voltage. :mrgreen:
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